Diffuse nitrate (NO3) contamination from intense agriculture adversely impacts freshwater ecosystems, and can also result in nitrate concentrations exceeding limits set in drinking water regulation, when receiving surface waters are used for drinking water production. Implementation of near-natural mitigation zones such as reactive swales or wetlands have been proven to be promising measures to reduce nitrate loads in agricultural drainage waters. However, the behavior of these systems at low temperatures and its dependence on systemdesign has not beenwell known until now. In this study, the behavior of a full-scale (length: 45 m) reactive swale treating drainage water from an agricultural watershed in Brittany (France), with high nitrate concentrations in the receiving river, was monitored for one season (6 months). As flow in this full-size field system is usually restricted to winter and spring months (December–May), it usually operates at lowwater temperatures of 5–10 WC. Tracer tests revealed shorter than designed retention times due to high inflows and preferential flow in the swale. Results show a correlation between residence time and nitrate reduction with low removal (<10%) for short residence times (<0.1 day), increasing to >25% at residence times >10 h (0.4 day). Performance was compared to results of two technical-scale reactive swales (length: 8 m) operated for 1.5 yearswith two different residence times (0.4 and 2.5 days), situated at a test site of the German Federal Environmental Agency in Berlin (Germany). Similar nitrate reduction was observed for comparable temperature and residence time, showing that up-scaling is a suitable approach to transferring knowledge gathered from technical-scale experiments to field conditions. For the design of new mitigation systems, one recommendation is to investigate carefully the expected inflow volumes in advance to ensure a sufficient residence time for effective nitrate reduction at low temperatures.

The wash out of agricultural auxiliary chemicals like fertilizer and pesticides via surface run-off or subsurface leaching into drainage systems or ground waters, which discharge into surface waters, presents an increasing risk for drinking water production and biodiversity in rivers and lakes. Mitigation zones are important measures to attenuate contamination at the source and relieve surface waters downstream. Under high flow conditions, as they occur during rainy seasons and snow melt, the effectiveness of such facilities is restricted due to bypass of untreated waters or very short contact times. This study of the Aquisafe 2 project focus on drainage water decontamination and examines mitigation zone designs with organic substrates for their potential to reduce a set of herbicides and nitrate (NO3-), concurrently and efficiently, at short hydraulic residence times (0.2 to 2.5 days) to prepare their implementation in contamination hot spots. The herbicides bentazone, atrazine and isoproturon were classified as most relevant for drinking water production. On the basis of comprehensive literature studies the organic substrates bark mulch and straw and the design of bioretention swales emerged to be of high potential for decontamination of drainage waters in mitigation zones. In laboratory scale studies the substrates were tested in degradation-, sorption- and leaching-experiments at temperatures around 21 °C for their potential to ensure long- lasting hydraulic permeability, denitrification and attenuation of the selected herbicides. The selected organic substrates provide a high and long term stable permeable conductivity to realize and maintain high flow. The effective porosity yielded around 0.45 and reduced within 1.5 years by only 25 %. Straw is a readily available organic carbon source, which can support effective and efficient denitrification at short hydraulic residence times. Bark mulch contains more resistant carbon species, but contributes also to NO3- removal. In mixture with straw the performance of bark mulch as organic carbon source for denitrification increases (co-metabolic decomposition). Organic substrates are characterized by strong wash out of dissolved organic carbon (DOC) and high denitrification rates (15 to 45 g-N m-3 d-1) in the start phase and successive decrease of denitrification performance due to loss of readily available organic carbon. Despite decline of performance, denitrification rates stabilized after one year of operation at constant conditions at a level of 4 to 10 g-N m-3 d-1 (10 to 25 % of input). The potential of the organic substrate to retain the selected herbicides is very different for each compound and bases on different dissipation paths. Denitrifying conditions are in general disadvantageous for retention of the selected herbicides. Bentazone is too persistent and mobile to be considerably retained under high flow conditions. Atrazine can be substantially removed from drainage waters. It is suspected to be attenuated predominantly by formation of bound residues at the organic substrate, especially bark mulch, and partially by degradation to hydroxy-atrazine. Isoproturon seems to be effectively retained under suboxic conditions by degradation to metabolites. At technical scale parallel retention of NO3- and atrazine and NO3- and isoproturon was investigated. The potential of the organic carbon source (mixture of bark mulch and

Diffuse nitrate (NO3) contamination from intense agriculture adversely impacts freshwater ecosystems, and can also result in nitrate concentrations exceeding limits set in drinking water regulation, when receiving surface waters are used for drinking water production. Implementation of near-natural mitigation zones such as reactive swales or wetlands have been proven to be promising measures to reduce nitrate loads in agricultural drainage waters. However, the behavior of these systems at low temperatures and its dependence on system design is not well known until now. In this study, the behavior of a full scale (length: 45 m) reactive swale treating drainage water of an agricultural watershed in Brittany (France) with high nitrate concentrations in the receiving river, was monitored for one season (6 months). As flow in this field scale system is usually restricted to winter and spring months (December – May), it usually operates at low water temperatures of 5°C - 10°C. Tracer tests revealed shorter than designed retention times due to high inflows and preferential flow in the swale. Results show a correlation between residence time and nitrate reduction with low removal (<10%) at short residence times (<0.1 d), increasing to >25% at residence times >10h (0.4 d). Performance was compared to results of two technical scale reactive swales (length: 8 m) operated for 1.5 years at two different residence times (0.4 and 2.5 days), situated at a test site of the German Federal Environmental Agency (UBA) in Berlin (Germany). Similar nitrate reduction was observed for comparable temperature and residence time, showing that up-scaling is a suitable approach to transfer knowledge gathered from technical scale experiments to field conditions. For the design of new mitigation systems, one recommendation is to investigate carefully expected inflow volumes in advance to ensure a sufficient residence time for effective nitrate reduction at low temperatures.

The present laboratory study tests the hypothesis that straw-bark mulch bioreactors are capable of concurrently retaining nitrate (NO3-) and the herbicides atrazine or bentazone at short hydraulic residence times (HRT). In a 1 year column experiment at HRT of ~4h three organic carbon sources, straw of common wheat (Triticum aestivum L.), bark mulch of pine tree (Pinus sp.) and a mixture of both materials, showed high reduction of continuously dosed NO3- (100mgL-1) with average denitrification rates of 23.4g-Nd-1m-3, 8.4g-Nd-1m-3 and 20.5g-Nd-1m-3, respectively. Under denitrifying conditions, fast and substantial retention of continuously dosed atrazine (20µgL-1) was observed with estimated dissipation times (DT50) between 0.12 and 0.49 days in the straw-bark mulch bioreactor. In parallel batch experiments, it could be confirmed that atrazine retention is based on adsorption to bark mulch and on degradation processes supplied by the organic materials as continual sources of carbon. In contrast, bentazone was not significantly reduced under the experimental conditions. While aging of materials was clearly observed in a reduction of denitrification by 60-70% during the experiment, systems still worked very well until the end of the experiment. The results indicate that the combined use of straw and bark mulch could increase the efficiency of mitigation systems, which are installed to improve the quality of drainage water before its release to surface waters. Further, the addition of these materials has the potential of parallel retention of NO3- and less mobile herbicides like atrazine, even during high flow events, as expected at the outlet of agricultural drainage systems. High removal is expected for mitigation system designed to remain saturated most of the time, whereas bioreactors that run periodically dry are not covered by this study. However, further experiments with the tested materials at technical or field scale under more realistic climate conditions need to be carried out.

In agricultural watersheds affected by diffuse pollution, limitation of fertilizer and pesticide application may not be sufficient to achieve good river water quality. After waterworks had to be closed in Brittany due to elevated nitrate concentrations in the river Ic (> 50 mg-NO3 L-1), the project Aquisafe has been initiated. The objective of Aquisafe is to reduce pollutant loads (nitrate and pesticides) from agricultural fields by implementation of near-natural mitigation zones at diffuse pollution hotspots at the head of watersheds. Simple and small solutions have to be designed in order to more efficiently reduce nitrate and pesticide concentrations in receiving rivers. In addition, a planning tool has to be developed to determine optimal locations to construct these systems. Finally, a tool to assess the effectiveness of these reactive zones on watershed water quality will be implemented. In order to reach the first objective, design features are tested on three scales: 1) laboratory scale, 2) technical scale and 3) field scale. 1) In the laboratory, column experiments were conducted with different organic substrates at short hydraulic residence times (HRT). The efficiency for parallel reduction of nitrate and two common herbicides in Europe, Bentazon and Isoproturon, was explored (Krause Camilo, 2012). 2) In technical scale, two parallel swales were filled with the most suitable material determined in (1) for a one year test. The influence of HRT and temperature was investigated. For nitrate, high reduction could be achieved at short HRT; results for herbicides still have to be confirmed. 3) One infiltration ditch and two simple wetlands were constructed in Brittany (France), taking into account experiences from other scales. These systems are now monitored to investigate the effects of upscaling. Site locations were chosen based on a validated and repeatable GIS-based overlay method that prioritises zones of potential contribution to nitrate pollution (Orlikowski et al, 2011). Additionally, a new wetland module is being developed for the Soil and Water Assessment Tool (SWAT). It allows to predict impacts of wetland constructions on nitrate concentrations in receiving rivers; the module is now implemented but still has to be calibrated with in situ monitoring results. The presentation will focus on results of the up-scaling approach, and will show how the tools of Aquisafe can be used for supporting the development of strategies at catchment scale.